In recent years, there are growing trends of high-speed processing and save of a large data with the rise of the digital technology. Consequently, a higher integration density and a higher performance are demanded in the semiconductor device equipped in the electronic equipment.
As to a semiconductor memory device, for example, in order to realize a higher integration density of DRAM (Dynamic Random Access Memory), the technology to employ a ferroelectric material or high-dielectric material as a capacitor insulating film of a capacitor element constituting DRAM, instead of the conventional silicon oxide or silicon nitride, is widely researched and developed.
Also, in order to realize a nonvolatile RAM that can execute a writing operation and a reading operation quickly at a lower voltage, the technology to employ a ferroelectric film having the spontaneous polarization characteristic as a capacitor insulating film is eagerly researched and developed. The semiconductor memory device having such ferroelectric capacitor insulating film is called a ferroelectric memory (FeRAM).
The ferroelectric capacitor stores the information by utilizing the hysteresis characteristic of the ferroelectric substance. The ferroelectric capacitor is provided to the ferroelectric memory, and the ferroelectric capacitor is constructed by putting the ferroelectric film between a pair of electrodes as the capacitor ferroelectric film. The ferroelectric film produces a polarization in response to an applied voltage between the electrodes, and keeps a spontaneous polarization even after the applied voltage is removed. Also, the polarity of the spontaneous polarization is inverted when the polarity of the applied voltage is inverted. Accordingly, information can be read by sensing this spontaneous polarization. The ferroelectric memory operates at a low voltage rather than a flash memory, and can execute a speedup writing while achieving a power saving.
The ferroelectric film constituting the capacitor of FeRAM is formed of lead zirconate titanate (PZT), PLZT formed by doping La in PZT, PLZT-based material in which Ca, Sr, or Si is micro-doped, a Bi-layer structure compound such as SrBi2Ta2O9(SBT,YI), SrBi2(Ta, Nb)2O9(SBTN,YZ), or the like. Such ferroelectric film is formed by the sol-gel method, the sputter method, the MOCVD (Metal Organic Chemical Vapor Deposition) method, or the like.
Normally, an amorphous or microcrystalline ferroelectric film is formed on the lower electrode by the above film forming method, and then the crystal structure is changed into the perovskite structure or the bismuth layer structure by the subsequent heat treatment.
As the electrode material of the capacitor, the material that is hard to oxidize or the material that can maintain conductivity even after oxidized must be employed. Commonly, either a platinum based metal such as Pt (platinum), Ir (iridium), IrOx (iridium oxide), or the like or its oxide is widely employed. Also, it is common that, as the interconnection material, Al (aluminum) is employed like the normal semiconductor device.
Like other semiconductor devices, a higher integration density and a higher performance are also required of FeRAM. In the future, a reduction of a cell area is needed. It has already been known that it is effective that the stacked structure should be employed in place of the conventional planar structure in reducing the cell area.
Here, the “stacked structure” denotes such a structure that a capacitor is formed directly over a plug (contact plug) formed on a drain of a transistor constituting a memory cell.
In the FeRAM having the stacked structure in the prior art, the capacitor has a stacked structure, in which a barrier metal, a lower electrode, a ferroelectric film, and an upper electrode are stacked in this order, directly over the plug formed of W (tungsten).
The barrier metal has a role to prevent oxidation of the W plug, and the material that fulfills both a function of the barrier metal and a function of the lower electrode is often employed. Therefore, it is impossible to separate clearly the barrier metal and the lower electrode material. Normally, the barrier metal and the lower electrode are formed by a combination of two films or more that are selected from titanium nitride (TiN) film, titanium aluminum nitride (TiAlN) film, iridium (Ir) film, iridium oxide (IrO2) film, platinum (Pt) film, and strontium ruthenium oxygen (SRO: SrRuO3) film.
The ferroelectric film constituting the ferroelectric capacitor is formed of an oxide. An oxygen defect is easily caused by the process in a nonoxidative atmosphere, and accordingly the characteristics of the ferroelectric film such as a quantity of inverted charges, a leakage current value, etc. are deteriorated. In order to recover the damage caused in the ferroelectric film, the heat treatment in an oxygen atmosphere must be applied plural times in manufacturing the ferroelectric capacitor. Therefore, a metal that is hard to oxidize in an oxygen atmosphere such as platinum, or the like, or a conductive oxide such as iridium oxide, ruthenium oxide, or the like is employed as the material of the upper electrode.
Meanwhile, the severe requests for miniaturization are imposed on the FeRAM recently, and accordingly miniaturization of the ferroelectric capacitor and employment of the multilayer interconnection structure are requested. Also, a lower voltage operation is requested in connection with the application to a mobile information processing device.
In order to make it possible for the FeRAM to operate at a low voltage, such a condition is requested that the ferroelectric film constituting the ferroelectric capacitor should have a large amount of inverted charges QSW. However, in step of forming the multilayer interconnection structure over the ferroelectric capacitor, the characteristics of the ferroelectric capacitor are deteriorated by the process applied in a reducing atmosphere or the process applied in a nonoxidative atmosphere.
More concretely explaining, when the upper electrode ob the ferroelectric capacitor is formed of the Pt film, the Ir film, or the like, a hydrogen in the reducing atmosphere used in forming the multilayer interconnection structure thereon enters into the Pt film, the Ir film, or the like, and then is activated by a catalytic action of the metal. Thus, such a problem arises that the oxide ferroelectric film in the ferroelectric capacitor is reduced by the activated hydrogen.
When the ferroelectric film is reduced, the operation characteristics of the ferroelectric capacitor are largely degraded. Such problem of the characteristic degradation of the ferroelectric film arises particularly conspicuously as the ferroelectric capacitor is miniaturized much more and thus the capacitor insulating film is miniaturized much more.
In JP2004-273787-A, in order to solve a reduction of a crystallinity caused due to an oxygen defect in the crystal and an excessive oxygen during a crystal growth simultaneously, such a method is set forth that an oxidizing gas of 40 vol % to 97 vol % should be employed in forming the IrO2 lower electrode.
In Japanese Patent No. 3661850, it is set forth that the upper electrode formed on the ferroelectric film should be constructed by a first conductive oxide film and a second conductive oxide film, and also the second conductive oxide film should be formed to have a composition that is closer to a stoichiometric composition than the first conductive oxide film, so that the ferroelectric capacitor can be miniaturized not to cause the degradation of the electric characteristics in the multilayer interconnection constructing steps.
In JP2006-128274-A, it is proposed that three layers of the upper electrode of the ferroelectric capacitor should be formed of platinum, iridium oxide, and iridium.
In JP2000-91270-A, such a method is disclosed that the lower electrode or the upper electrode should be formed successively of the Ir film and the IrO2 film. Also, such a method is disclosed that, in order to reduce the voids in the ferroelectric film, RTA (Rapid Thermal Annealing) should be applied after the IrO2 film is formed and then the Ir film should be formed.
In Japanese Patent No. 3299909, such an electrode constructed by the stacked structure is set forth that the IrO2 film of 36 nm to 83 nm thick is used as the upper layer and the Ir film of 22 nm to 66 nm thick is used as the lower layer.
In JP2001-127262-A, such a two-step sputter method is disclosed that the IrO2 film is formed at a low power and then the IrO2 film is formed at a high power.
In JP2002-246564-A, JP2005-183842-A, etc., steps of (i) forming the film of the conductive lower electrode made of a noble metal, (ii) covering the lower electrode with the ferroelectric material layer, (iii) applying Rapid Thermal Annealing (RTA) to the ferroelectric layer for the first time, (iv) forming the film of the upper electrode layer made of a noble metal oxide, and then (v) applying the annealing to the ferroelectric layer and the upper electrode layer for the second time are set forth. It is set forth that a larger quantity of switching charges can be obtained according to such steps and also the good fatigue characteristic can be obtained preferably.
In JP2005-183842-A, such a method is also disclosed that RTA is applied to the conductive oxide film formed on the ferroelectric film and then the furnace annealing is applied in the oxygen atmosphere.
In JP2006-73648-A, upon forming the upper electrode film on the ferroelectric film, steps of forming the IrOx film containing the crystallized microcrystals and then forming the IrOx film containing the columnar crystals are disclosed. Accordingly, it is set forth that, even when the ferroelectric film is formed as a thin film, the characteristic of the ferroelectric film can be extracted sufficiently.
In JP2003-204043-A, such a method is proposed that the ferroelectric film is formed, and then the IrOx film of 150 to 250 nm thick is formed thereon as the second conductive film by the sputtering method.
In JP2006-245457-A, it is proposed that formation of the conductive film on the side wall of the capacitor in etching the stacked film is prevented by setting a film thickness of the stacked film of Ir and IrO2 constituting the capacitor lower electrode below 100 nm, and thus a leakage current between the capacitor upper electrode and the capacitor lower electrode is reduced.